Mountains are classified by how they are formed. Mountains form by volcanic eruption, heat and pressure, and the movement of the earth's plates. A mountain's shape and location give clues to how it was formed and to its classification.
Volcanic mountains form from the hardened lava produced during volcanic eruptions. The lava piles up, along with ash and rock, over time to form the mountain. Dome mountains form from heat and pressure building up under the crust and pushing straight up. Plateau mountains form when two tectonic plates in the crust push together, moving the land straight up to where they collide. Fault-block mountains form when plates push together, causing the crust to break. Portions of the crust push up, often with sheer sides, and other portions slip underneath.